Understanding the Effects of Cationic and Anionic Substitutions in Spinel Cathodes of Lithium-ion Batteries

The global demand for energy has reached epic proportions and is projected by the U.S Energy Information Administration (EIA) to continue to grow in the future. Efficient storage and utilization of electrical energy is critical if we are to exploit renewable energies like solar and wind to their full potential. In this regard, lithium-ion batteries are being intensely pursued as energy storage devices because they provide higher energy and power densities compared to other battery systems such as lead-acid and nickel metal – hydride batteries. In addition, they are also being pursued as a power source for transportation applications. Cost, safety, cycle life, and power capability are important criteria for these large battery applications. LiMn2O4 spinel as a cathode for lithium-ion batteries is appealing in this regard due to its lower cost and fast charge capability. Yet severe capacity fade has plagued the spinel cathode and prevented it from becoming widely commercialized. Many different approaches have been taken to improve the capacity retention of the spinel cathode. Our group has shown that substituting other transition metals for manganese followed by partial substitution of fluorine for oxygen can improve the capacity retention while maintaining moderate capacity values. We present here a systematic investigation of (i) the amount of fluorine that can be substituted for oxygen before impurity phases form or performance degradation occurs and (ii) how the chemical characteristics of the dopants (electronegativity and dopant-oxygen bond dissociation energy) affect the electrochemical performance of the cathode materials.

Understanding the Effects of Cationic and Anionic Substitutions in Spinel Cathodes of Lithium-ion Batteries

The global demand for energy has reached epic proportions and is projected by the U.S Energy Information Administration (EIA) to continue to grow in the future. Efficient storage and utilization of electrical energy is critical if we are to exploit renewable energies like solar and wind to their full potential. In this regard, lithium-ion batteries are being intensely pursued as energy storage devices because they provide higher energy and power densities compared to other battery systems such as lead-acid and nickel metal – hydride batteries. In addition, they are also being pursued as a power source for transportation applications. Cost, safety, cycle life, and power capability are important criteria for these large battery applications. LiMn2O4 spinel as a cathode for lithium-ion batteries is appealing in this regard due to its lower cost and fast charge capability. Yet severe capacity fade has plagued the spinel cathode and prevented it from becoming widely commercialized. Many different approaches have been taken to improve the capacity retention of the spinel cathode. Our group has shown that substituting other transition metals for manganese followed by partial substitution of fluorine for oxygen can improve the capacity retention while maintaining moderate capacity values. We present here a systematic investigation of (i) the amount of fluorine that can be substituted for oxygen before impurity phases form or performance degradation occurs and (ii) how the chemical characteristics of the dopants (electronegativity and dopant-oxygen bond dissociation energy) affect the electrochemical performance of the cathode materials.

Dr. Iyer, thank you for the questions. Ideally we would like to have no capacity fade occur, but I think that you are asking about the acceptable level for commercialization. Industry usually defines “End of Life” for the battery system the point at which you only obtain 80% of the starting capacity. This is measured by the number of cycles it takes to reach this lower limit. Depending on what the specific chemistry of the cathode is and environment it is being used in (high temp, fast charge, etc.), this can vary quite a bit (i.e. 500 – 2000 cycles). I would say that some spinel chemistries have already met the capacity fade limit since it has found application in the Nissan leaf and Chevy volt. Normally though, we also consider the spinel cathode a viable competitor to the layer oxide cathode because the spinel is manganese based and manganese is less toxic than cobalt, which is found in the layered oxide. Additionally, the spinel has very fast kinetics due to its 3D lithium transport within the structure. This allows for fast charge/discharge of the battery. Although there are several applications that would benefit from these desirable characteristics, EV and PEV applications seem to be at the top of the priority list. If we want our infrastructure to change in order to EV’s and PEV’s to really take hold in the U.S., then the spinel has some of the characteristics to move us in the right direction. In addition, because the spinel cathode has high power capability, it can be used for power tools, etc. I hope this answers your questions. Please let me know.

Dr. Clancy, with the stoichiometry that we tested (Li1.1Mn1.8M0.1O4) I would recommend Ni, with a caveat. There is no elixir dopant. Each brings its advantages. The caveat with Ni as a dopant is that the cathode capacity is less because it is a divalent cation. This would require more cathode material (and higher cost generally) for an application because of the lower capacity. If the dopant is trivalent, then the theoretical capacity increases. Still, Ni reduces the capacity fade to an acceptable level for the spinel.

Dr. Koodali, the porosity and surface area of cathode materials do play a role. They are very much related as well. For instance, you can increase the surface area of a material by increasing the porosity. An increase in surface area can lead to better performance, especially if the materials electronic conductivity is not very good (i.e. LiFePO4). The challenges with higher surface area come from dissolution of the material and larger SEI/surface area ratios. Both of these would worsen the performance of the cathode material. The spinel cathode exhibits higher ionic and electrical conductivity when compared to the phosphate (LiFePO4) so the particle size doesn’t generally need to be smaller. In fact, because manganese dissolution into the electrolyte is a known problem for spinel cathodes, it is more desirable to have medium-large size particles for the spinels. For our samples we observed no change in the morphology or surface area upon ionic substitution.

I’m afraid I didn’t see in your poster what the capacity and capacity fade was for Li batteries with the pure LiMn2O4 spinel cathode. Could you comment on the relative improvement that the F substitutions and cation substitutions in the cathode make on the desirable Li battery performance characteristics?

Dr. Harrison, thanks for the question. The capacity fade in LiMn2O4 is associated with the concentration of Mn3+ in the material. Mn3+ has a d4 electronic configuration and is associated with Jahn-Teller distortion which is a cause for structural instability. In addition, it has been shown that when the concentration of Mn3+ is high in the material, more manganese dissolves into the electrolyte. By substituting other cations in for Mn, you lower the concentration of Mn3+ and reduce the structural and chemical instability. The downside of this is that Mn3+ is the redox center (i.e. the more Mn3+ the higher the capacity). So by trying to improve the cyclability through cation doping we reduce the capacity. The advantage of fluorine doping is that it increases the Mn3+ concentration again but it also reduces the structural instability during cycling.

Thanks Arturo. I’m sorry that I didn’t phrase my question better. It really concerns whether there are absolute ways to quantify or define “capacity” for a cathode material and then how does the regular LiMn2O4 spinel cathode’s capacity (& capacity fade) compare to those of the doped ones? Presumably, these are the comparisons that a battery designer needs.

Dr. Harrison, the capacity is measured in mAh/g. I think that is what you are asking for. The undoped spinel theoretical capacity is higher than the undoped. For example for our study the theoretical capacity of our materials is ~ 90 mAh/g (for trivalent dopants) and ~ 75 mAh/g (divalent dopants). The theoretical capacity of the undoped spinel is ~150 mAh/g. The difference in the theoretical capacity is all related to the changing concentration of Mn3+ (discussed in previous post). In 50 discharge cycles, the capacity fade for the undoped spinel is ~ 35% of the initial capacity. In comparison, the capacity fade in 50 cycles in the doped spinel would be ~ 5% on the high end (i.e. the Al doped sample in our work). I hope this answers your question. Thanks for the follow up.

You state that the LiMn2O4 cathodes are attractive due to low cost and fast charge capability. Presuming that you can achieve the needed capacity fade reduction, what would stand in the way of commercialization? Can one just drop in a new cathode type without having a cascade effect on the other components and device fabrication conditions?

Dr. Yates, thank you for the questions. The capacity fade has been controlled sufficient to begin commercialization in the Nissan Leaf and Chevy Volt, although there are other challenges or barriers to a full exploitation of the spinel cathode. The cost is low for the spinel, but the capacity is also low. So in general you would need more spinel material to provide the same capacity as other cathodes. This may be the reason that we are now finding out that Nissan and Chevy used a composite cathode with layered oxide and spinel material. In answer to your second question, I would say that to substitute spinel in place of the most commercialized cathode (layered oxide) would not affect the other components in a negative way. Much of the research that has been done on the spinel has included similar components used commercially, with probably the exception of the anode. Commercially, graphite is used as the anode and in the research community lithium is generally used. Still, because of the similar voltage ranges for LiCoO2 and LiMn2O4, the formation of the SEI layer would be similar as some research has shown. The fabrication would also be similar. Both layered oxide and spinel materials can be made through solid state reaction (which is cheaper than other techniques).

johnny simms

Guest

May 22, 2013 | 12:14 a.m.

Do you have any excessive heating problems with this kind of energy storage? How fast can this battery be charged? Can this storage medium be multiplied to create a base load energy storage solution for installation in a Power Substation. If so, how large (MW) do you envision a base load battery to be able to be built? how many charge discharge cycles can this battery handle before the battery begins to loose its ability to hold a charge. How does the Watts/pound for this battery compare to gasoline and to other battery types?

Johnny, thanks for watching my video! You may have heard about thermal runaway, well this is a problem with lithium ion batteries. Once a certain temperature is reached the cell becomes unstable and the temperature continues to increase, possible leading to explosion. A lot of safety measures are generally put in place to avoid this (unless you are Boeing, that is why they had their dreamliner recalled for 5 months). Let me answer the questions about the cycle life. The spinel can hold up to 80 % of its initial capacity up to 2000 cycles. The power density of lithium ion batteries ranges from 300-1500 W/kg. The typical car engine has a power of 150-400 W/kg. I think that we generally want to compare the specific power of the gasoline but the limitation of the Carnot heat engine needs to be considered. Let me answer your other two questions in another post.

Batteries can be multiplied to create a base load. Since base load plants are usually in the MW I am not sure that it is feasible to meet that with batteries, but I may be wrong. The cost of the batteries and the lower power capability would just not work so well. I have heard of research where they are trying to meet the peak demand with community level energy storage.

Eric Ericson

Kathy Sherman

Guest

May 22, 2013 | 06:49 p.m.

I thank you for the term energy addict.ion and for the map of it. I have previously seen only the rather shocking picture of lights out in the mid-Atlantic when the grid went down. That does say it is not just the main source of petroleum use and air pollution, i.e., transport. How about dimming the office lights before sacrificing rural America to Wind Energy and its nighttime sky pollution and 24/7 noise pollution (as for fracking)?